Recently, an ionomer resin has widely been used for the cover of a golf ball (e.g. Japanese Patent Publication Sho 49-49727). Particularly, the ionomer resin is exclusively used in a two-piece solid golf ball composed of a solid core and a cover covering the solid core, because the ionomer resin is superior in processability, durability, cut resistance and rebound characteristics compared to other reins used for golf ball covers.
However, the ionomer resin is poor in shot feel and controllability (ease of putting spin on the ball) compared to a balata cover (transpolyisoprene) which has been used as a cover for a thread wound golf ball because ionomer resins are too hard and too rigid. In a golf ball using an ionomer resin for the cover, a sharp metallic "click" sound emits at the tie of contacting the ball with a golf club. This sharp metallic "click" sound is shunned by professional golfers and high-level amateur golfers, because such a sound is unpleasant to the professional golfer or to high-level amateur golfers.
Japanese Laid-Open Patent Publication Hei 1-308557 suggests that an ionomer resin is softened by blending (mixing) a soft ionomer resin of a sodium or zinc salt of a terpolymer comprising an olefin having 2 to 8 carbon atoms, an unsaturated monocarboxylic acid having 3 to 8 carbon atoms and an acrylate having 2 to 22 carbon atoms with an ionomer resin to improve the shot feel and controllability. However, when the above soft ionomer resin is so blended, the shot feel and controllability are improved but the flight performance and cut resistance are adversely degraded.
Japanese Laid-Open Patent Publication Hei 5-220240 suggests that an ionomer resin is softened by blending a glycidyl group-containing polymer therewith. However, satisfactory results have not been obtained in flight performance, because the selection of the a base polymer of the glycidyl group-containing polymer is difficult.
In addition to the above trials, various trials of softening the ionomer resin to improve shot feel and controllability have been made, but satisfactory results have not been obtained at present. Accordingly, a golf ball having satisfactory performances is still desired.
On the other hand, the thread wound golf ball is basically composed of a solid or liquid center, a thread rubber layer formed by winding a thread rubber around the center and a cover for covering the thread rubber layer. Such a ball is excellent in shot feel and controllability in comparison with the two-piece solid golf ball using a solid core The thread wound golf ball, however, is poor in flight distance in comparison with the two-piece solid golf ball because of the large amount of spin and the low launch angle.